The present invention relates to a device for perforating ventilation holes in cigarettes or similar. For producing so-called "ventilated" cigarettes, perforating devices are known to be employed, the "piercing" tool of which consists of a laser source. Italian Patent No. 1.168.682 filed on 12 Sep. 1983 by G. D. S.p.A. relates to a device for perforating ventilation holes in cigarettes supported inside respective seats mounted for rotation on a conveyor drum turning about its axis U.S. Pat. No. 4,633,891 corresponds to the Italian patent.
According to said Italian Patent, the perforating device comprises a laser source for emitting a laser beam along the rotation axis of the conveyor drum; and an optical reflecting and focusing system for directing and focusing the beam emitted by the laser source on to specific surface points of each cigarette as this turns about its axis inside the respective said seat.
For this purpose, the optical system comprises a mobile unit turning about the rotation axis of the conveyor drum and in turn comprising two mirrors, a first of which is located on said rotation axis for directing the beam outwards, and a second of which receives the beam reflected by the first and directs it inwards towards said rotation axis and on to a fixed reflecting element. The reflecting element presents a ring of mirrors equally spaced about the rotation axis of the conveyor drum, for successively intercepting the beam and directing it outwards on to the cigarettes.
Prior to reaching the fixed reflecting element, the beam deflected inwards by the second mirror passes through a focusing lens which, in the absence of the reflecting element, would focus the beam deflected by the second mirror on to the same point on the rotation axis of the drum.
On the known device described above, the speed ratio of the drum and mobile unit is so fixed that one full turn of the mobile unit about is axis corresponds to a one-step displacement of the cigarettes, i.e. the distance between two adjacent cigarettes on the conveyor drum. Consequently, if n is the number of holes to be perforated in each cigarette, the cigarettes are perforated as they are fed by the drum over an arc of n steps in length, along which arc each cigarette makes one full turn about its axis by virtue of the motion imparted by the rotary seat in which it is housed.
The reflecting element presents n mirrors arranged over an arc the center angle of which coincides with that of the perforating arc.
For each complete turn of the mobile unit about its axis, the above device perforates a hole in each cigarette on the perforating arc, so that, on exiting said arc, each cigarette presents a ring of n holes equally spaced about its circumference.
This conforms with a well-known optical law whereby, if a beam is focused on to a given point by a focusing device, and said point remains stationary despite said focusing device being moved along an given trajectory, any deflection of the focused beam results in the formation of a "reflected focus" which, like the original, remains stationary alongside a change in position of the focusing device.
If the known perforating device described above is examined in the light of the above principle and the fact that, in the absence of the mirrors on the reflecting element, the beam is focused on to the same fixed point on the axis of rotation of the drum, it follows that, via the interposition of a mirror on the reflecting element, the beam is focused on to a further point, which remains stationary for as long as it takes the beam to sweep the mirror, and then moves rapidly on to a further fixed point as the beam begins to sweep the next mirror on the reflecting element.
On the known device described above, the beam is thus "pulsated" by the mirrors on the reflecting element, the effect of which is to cyclically locate the beam focus on the n reflected focuses arrange over and in a fixed position in relation to the perforating arc.
If a strict speed ratio is maintained between the conveyor drum and mobile unit so that, as already stated, a complete turn of the mobile unit corresponds to one-step displacement of the drum, and if the drum is so "timed" to the mobile unit that, when produced, each of said reflected focuses coincides with a surface point on a respective cigarette, the known device described above provides, at each reflected focus, for perforating the surface of the cigarette currently coinciding with the position of said reflected focus.
As each cigarette moves through all the said points as it travels over the perforating arc, a total of n holes are perforated by the time the cigarette exits the perforating arc.
Moreover, as each cigarette, as it travels over the perforating arc, makes a complete turn about its axis at constant speed, said holes are equally distributed about the circumference of the cigarette.
Clearly, therefore, the perforating system referred to in Italian Patent No. 1.168.682 is extremely rigid, in that the number of holes to be perforated in each cigarette, depending as it does on the number of mirrors on the fixed reflecting element, must be determined beforehand, and a strict speed ratio, both in terms of absolute speed and timing, must be maintained between the mobile unit and drum. In fact, even the slightest variation in said speed ratio in the formation of reflected focuses not coinciding with respective surface points on the cigarettes and, consequently, in total inefficiency of the perforating system as a whole.